Key Information about I-131

Nuclear bomb detonation produces a small amount of local I-131 fallout. Most I-131 distributes over large distances, typically with only 10% making its way to the earth's surface before undergoing spontaneous radioactive decay to stable xenon-131.

Ingestion: Where I-131 is not inhaled, ingestion may represent a more significant route of internal contamination. Doses to humans from inhalation or from ingestion of plants, animals, or water, however, are usually small in comparison to that from milk intake (see below).

Historically, milk consumption has been a more significant route of internal contamination than inhalation, both in terms of numbers of individuals affected and internal dose. (Illustration) Deposition of I-131 on pasture grasses, followed by I-131 ingestion by cows or goats and then human ingestion of contaminated milk and fresh dairy products may occur. The concentration of I-131 in milk from goats and sheep is 10 times higher than the concentration in cow's milk.

After I-131 ingestion, the most critical dietary information needed is

How and When KI Protects

KI protects only the thyroid from radioactive iodine uptake. KI does not provide protection for any other organs.

KI is effective as a radiation medical countermeasure only for internal contamination with radioactive iodine. KI does NOT prevent or treat problems due to internal contamination from any other isotope.

When to take KI

KI is highly effective if taken several hours before inhaled contamination with radioactive iodine.

Taking KI within 1 to 2 hours after inhalation of I-131 can block more than 90% of the radioactive iodine uptake by the thyroid.

If KI is taken more than 4 hours after inhalation of I-131, much less thyroid uptake of I-131 is blocked.

KI is even less effective at preventing radioiodine uptake by the thyroid if taken more than 12 hours of a time-limited contamination episode.

Since the protective effect of a single dose of KI lasts approximately 24 hours, repeat KI administration may be indicated for some members of the public where contamination is continuing or ongoing.

Figure 1 below shows the effectiveness of thyroid blocking achieved by administering stable iodine at different times before (only at 1.2 hours) or various times after a 4-hours intake (inhalation) of I-131.

It is a conceptual graphic that should NOT be used clinically.

Note that the y-axis is in arbitrary units of averted dose, not a specific dose.

The higher the number on the y-axis the more dose is averted.

1.0 is 100% dose to thyroid averted.

Dietary iodine intakes in the caption of Figure 1 refers to intake of stable KI, not I-131.

Duration of Therapy

If primary public health protection measures (evacuation, sheltering, and control of the food supply) cannot be readily put into place, multi-dosing of KI may be required, sometimes up to 7-14 days.

Incident managers and public health experts will provide advice appropriate to the radiation event.

Once the plume has passed and/or public health protection measures (including distribution of KI) have been put place, prevention of internal contamination with radioiodines is best accomplished by food control measures and NOT by repeated dosing with KI.

Because radioactive iodine has a short half-life, grain products and canned milk or vegetables from sources affected by radioactive fallout, if stored for weeks to months after production, pose no radiation risk. Thus, late KI prophylaxis at the time of delayed consumption is not required.

Contraindications to KI

A seafood or shellfish allergy does not necessarily indicate allergy to iodine.

KI should not be given to individuals with certain skin disorders (such as dermatitis herpetiformis or hypocomplementemic or urticaria vasculitis).

KI should be used with caution and with careful medical monitoring in individuals with thyroid disease (such as multinodular goiter, Graves disease, and autoimmune thyroiditis), especially if dosing extends beyond a few days. Such individuals should have monitoring of thyroid function.

Radioactive Iodine in the Problem of Radiation Safety, Moscow, Atomizdat, 1972, editor L.A. Il'in, translated from Russian and published in English by the US Atomic Energy Commission, Office of Information Services in 1974 as document AECF-tr-7536, see especially page 224.)